1. Field of the Invention
The present invention is in the field of thermal treatment of mineral raw materials for the manufacture of cement clinker and involves the use of separate preheating systems operating in parallel and the recirculation of exhaust gases into at least one of the systems for efficient operation.
2. Description of the Prior Art
In DE-OS No. 29 33 289 there is disclosed a cement manufacturing facility using a parallel connection of two preheating systems, one of which is a traveling grate and the other a cyclone heat exchanger. The outputs of these two systems are used to charge a rotary tubular kiln with the raw materials pretreated therein, the rotary tubular kiln being followed by a clinker cooler. Both the traveling grate and the cyclone heat exchanger are equipped with devices for the introduction of additional fuel, these devices being disposed in the gas conduits connecting these units to the clinker cooler. Fuel in lumpy form such as low-grade coal, scrap tires, household refuse, and the like, is added to the raw materials as they enter the rotary tubular kiln. The exhaust gases of the clinker cooler thus serve as a source of combustion air for the rotary tubular kiln, the traveling grate, and the cyclone heat exchanger. The exhaust gases of the traveling grate are eliminated from the system, and the exhaust gases from the cyclone heat exchanger are directed to a grinder/drier system for the cement raw materials to be thermally treated. The purpose of the systems described in that patent is to increase performance of an existing traveling grate/rotary tubular kiln/cooler installation by providing a further preheating system with precalcination capability, namely, a cyclone heat exchanger to the traveling grate.